Method of forming ferroelectric thin film

ABSTRACT

There is provided a method of forming a ferroelectric thin film of vinylidene fluoride homopolymer having crystal form I which is applicable to various substrates in relatively easy way (coating conditions, application method, etc.). The method of forming a ferroelectric thin film comprising vinylidene fluoride homopolymer comprises the step (i) for preparing a green powder of vinylidene fluoride homopolymer comprising crystal form I alone or as main component by subjecting vinylidene fluoride to radical polymerization in the presence of a radical polymerization initiator, the step (ii) for forming a thin film on a substrate surface by using vinylidene fluoride homopolymer which comprises crystal form I alone or as main component and is obtained from the green powder product of vinylidene fluoride homopolymer comprising I-form crystal structure alone or as main component, and the step (iii) for subjecting the thin film of vinylidene fluoride homopolymer formed in the step of above (ii) to polarization.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation-in-part of PCT international application No.PCT/JP2004/004020 filed on Mar. 24, 2004, incorporated herein byreference.

BACKGROUND OF THE INVENTION

The present invention relates to a method of forming a ferroelectricthin film of vinylidene fluoride homopolymer.

Polymer type ferroelectric materials have advantages such asflexibility, light weight, good processability and low price as comparedwith inorganic ferroelectric materials such as ceramics. There areknown, as represented examples thereof, vinylidene fluoride polymerssuch as polyvinylidene fluoride (PVdF) and vinylidenefluoride/trifluoroethylene (VdF/TrFE) copolymer.

With respect to PVdF, crystal structures thereof are roughly classifiedinto three kinds such as I-form (also said to be β-form), II-form(α-form) and III-form (γ-form). Among them, it is only I-form crystalthat can exhibit sufficiently high ferroelectricity.

PVdF having a high molecular weight which is prepared by radicalpolymerization method forms crystal form II and does not exhibitferroelectricity as it is. In order to convert the crystal form II ofPVdF to crystal form I, there are required complicated post-steps suchas stretching and heat-treating of a film or rapid cooling under highpressure at casting.

Matsushige et al have studied formation of thin film of vinylidenefluoride oligomer having crystal form I by using vinylidene fluorideoligomer: CF₃(CH₂CF₂)_(n)I (number average degree of polymerizationn=17) having crystal form II, and in this study, have found that a thinfilm of vinylidene fluoride oligomer of crystal form I was formed onlyby deposition coating on a KBr substrate at a substrate temperatureTs>0° C., for example, at 25° C. Also it was found that indeposition-coating on a KCl substrate at 25° C., a coating filmcontaining a mixture of I-form and II-form crystals was formed and wasalmost converted to a thin film of vinylidene fluoride oligomer ofcrystal form I by heat-treating (at a temperature of not less than 110°C.) after the coating. However, in a substrate (SiO₂, Pt, Au, or thelike) having small interaction with vinylidene fluoride oligomer, justafter the deposition, and further even after the following heattreatment, only thin films containing a mixture of I- and II-formcrystals have been obtained (M & BE Vol. 11, No.2, 145 (2000)).

Also Matsushige et all have recently found that a thin film ofvinylidene fluoride oligomer of crystal form I could be formed onvarious substrates by deposition of vinylidene fluoride oligomer ofcrystal form II under extremely low temperature environment of not morethan −130° C. (Polymer Preprint, Japan, Vol. 51, No. 12, 3097 (2002)).

As mentioned above, thin films of I-form crystal have not been formedonly by coating at room temperature except coating on KBr substrate.

Okui et all have made analysis of crystal structure with respect tovinylidene fluoride oligomer: CCl₃(CH₂CF₂)_(n)Cl (number average degreeof polymerization n=9) prepared by radical polymerization by using CCl₄as a chain transfer agent (telogen) and dinormalperoxy dicarbonate as acatalyst, and have reported that this oligomer was a mixture of crystalform I (β-form) and crystal form III (γ-form) and had a crystallinemelting point Tm at two points (74° C. and 110° C.) (Polymer Journal,Vol. 30, No. 8, pp. 659 to 663 (1998), and POLYMER Vol. 38, No. 7, pp.1677 to 1683 (1997)).

In this case, thin films were formed by casting or the like for thepurpose of analyzing crystal structures. However, the thin films wereformed simply in a process of structural analysis, and no studies havebeen made with respect to polarization intended to obtainferroelectricity and resultant ferroelectric characteristics.

In the mentioned process for preparing vinylidene fluoride oligomer byusing CCl₄ as a chain transfer agent (telogen), a molecular weightdistribution of the obtained vinylidene fluoride oligomer is wide, andeven if crystal forms I (β-form) are obtained, the crystal structureseasily become a mixture of I-form (β-form) with II-form (α-form) andIII-form (γ-form) and a purity of crystal form I (β-form) becomes low,which lowers ferroelectric characteristics of thin films formed by usingthe obtained vinylidene fluoride oligomer even if polarization iscarried out.

By the way, there are various known processes for preparing polymersusing vinylidene fluoride monomer.

For example, “iodine transfer polymerization process” usingperfluoroalkyl iodide as a chain transfer agent (or telogen) is known,and a molecular weight distribution of high molecular weight polymer canbe made narrow particularly in non-crystalline polymers used forfluorine-containing rubbers or the like such as vinylidenefluoride/hexafluoropropene copolymer and vinylidenefluoride/tetrafluoroethylene/hexafluoropropene copolymer (KobunshiRonbunshu, 49(10), No. 10, pp. 765 to 783 (1992)).

In addition, there are known other polymerization processes such as apolymerization process not using a chain transfer agent and apolymerization process using a hydrocarbon chain transfer agent(telogen) such as isopentane or alcohol. However there is a problem thata molecular weight distribution of the obtained polymer becomes wide,and a purity of crystal form I (β-form) is lowered like theabove-mentioned preparation processes.

Also with respect to polymerization processes for a low molecular weightvinylidene fluoride polymer, there are disclosed a process using, as atelogen, the same perfluoroalkyl iodide as above (JP56-57811A), aprocess using alcohols as a telogen (JP59-117503A) and a process usingperfluoroalkyl bromide as a telogen (JP63-93736A, JP7-179523A, etc.).

However all of those preparation processes are intended to preparevinylidene fluoride copolymers (copolymer oligomers), and studies onvinylidene fluoride homopolymers (homopolymer oligomers) havingcrystallinity have not been made. Furthermore there are disclosed noprocesses for efficiently preparing, at high purity, vinylidene fluoridehomopolymers having crystal form I (β-form) which can exhibitferroelectric characteristics.

As mentioned above, vinylidene fluoride homopolymer comprising crystalform I alone or as main component has not yet been obtained as a greenpowder product just after polymerization, and as a matter of course,there is no example of production of ferroelectric thin film using thegreen powder product of vinylidene fluoride homopolymer comprisingcrystal form I alone or as main component.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method of forming aferroelectric thin film of vinylidene fluoride homopolymer which isapplicable to various substrates in a relatively easy manner (coatingconditions, application method, etc.).

The present inventors have made intensive studies and as a result,obtained vinylidene fluoride homopolymer having crystal form I directlyby polymerization. The present inventors have further found that a thinfilm of vinylidene fluoride homopolymer having crystal form I which canexhibit ferroelectricity, can be formed on various substrates by usingthe obtained vinylidene fluoride homopolymer having crystal form I evenby usual coating method or coating conditions, for example, on a siliconwafer by spin coating at room temperature, and have found that bysubjecting this thin film to polarization treatment, a polymer typeferroelectric thin film can be produced.

Namely, the present invention relates to a method of forming aferroelectric thin film comprising vinylidene fluoride homopolymer whichcomprises the following steps (i), (ii) and (iii).

-   (i) A step for preparing a green powder product of vinylidene    fluoride homopolymer comprising crystal form I alone or as main    component by subjecting vinylidene fluoride to radical    polymerization in the presence of a radical polymerization    initiator.-   (ii) A step for forming a thin film on a substrate surface by using    vinylidene fluoride homopolymer which comprises crystal form I alone    or as main component and is obtained from the green powder product    of vinylidene fluoride homopolymer comprising crystal form I alone    or as main component.-   (iii) A step for subjecting the thin film of vinylidene fluoride    homopolymer formed in the above step (ii) to polarization treatment.

The method of forming a ferroelectric thin film of the present inventionmay further include a step (iv) for heat-treating the thin film ofvinylidene fluoride homopolymer at a temperature of not less than 50° C.and lower than a crystalline melting point of the vinylidene fluoridehomopolymer.

In the vinylidene fluoride homopolymers comprising crystal form I aloneor as main component, when attention is given to proportions of therespective vinylidene fluoride homopolymers comprising crystal form I,II or III in the green powder product of vinylidene fluoride homopolymerwhich are calculated by IR analysis, it is preferable that theproportion of vinylidene fluoride homopolymers having crystal form Isatisfies both of (Equation 1):100≧I-form/(I-form+II-form)≧50% by weight  (Equation 1)and (Equation 2):100≧I-form/(I-form+III-form)≧50% by weight  (Equation 2)and it is further preferable that the proportion of vinylidene fluoridehomopolymers having crystal form I satisfies both of (Equation 3):100≧I-form/(I-form+II-form)≧70% by weight  (Equation 3)and (Equation 4):100≧I-form/(I-form+III-form)≧70% by weight  (Equation 4).

It is preferable that a number average degree of polymerization of thegreen powder product of vinylidene fluoride homopolymer comprisingcrystal form I alone or as main component is from 4 to 20.

The step (ii) for forming a thin film of vinylidene fluoride homopolymermay be carried out by applying a liquid composition containing thevinylidene fluoride homopolymer on a substrate surface or by vacuumvapor deposition of a composition containing the vinylidene fluoridehomopolymer on a substrate surface.

It is preferable that the step (ii) for forming a thin film ofvinylidene fluoride homopolymer is carried out at a temperature of notless than 10° C. and lower than a crystalline melting point of thevinylidene fluoride homopolymer.

The thin film of vinylidene fluoride homopolymer may be formed on asurface of silicon substrate or on a surface of metallic substrate, forexample, on a surface of at least one selected from the group consistingof aluminum, copper, gold, silver and platinum.

The vinylidene fluoride homopolymer comprising crystal form I alone oras main component can be formed into a thin film on a substrate surfacein a thickness of from 0.1 to 1,000 nm.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an IR chart of vinylidene fluoride homopolymer of all-I-formcrystal structure.

FIG. 2 is an IR chart of vinylidene fluoride homopolymer of all-II-formcrystal structure.

FIG. 3 is an IR chart of vinylidene fluoride homopolymer comprising amixture of crystal forms I and II for explaining a method of readingpeak heights of characteristic absorption of crystal forms I and II.

FIG. 4 is an IR chart of vinylidene fluoride homopolymer comprising amixture of crystal forms II and III for explaining a method of readingpeak heights of characteristic absorption of crystal forms II and III.

FIG. 5 is an IR chart of vinylidene fluoride homopolymer comprising amixture of crystal forms I, II and III, in which a content F(I) ofcrystal form I is known, for explaining a method of reading peak heightsof characteristic absorption of crystal forms I and III.

FIG. 6 is an IR chart of vinylidene fluoride homopolymer of all-I-formcrystal structure which is obtained in (1-1) of Preparation Example 1.

FIG. 7 is an IR chart of vinylidene fluoride homopolymer comprising amixture of crystal forms I and III which is obtained in (1-8) ofPreparation Example 1.

FIG. 8 is an IR chart of vinylidene fluoride homopolymer comprising amixture of crystal forms II and III which is obtained in (2-1) ofPreparation Example 2.

FIG. 9 is an IR chart of vinylidene fluoride homopolymer comprising amixture of crystal forms I and II which is obtained in (3-1) ofPreparation Example 3.

DETAILED DESCRIPTION

The method of forming a ferroelectric thin film of the present inventionis, as mentioned above, a method of forming a ferroelectric thin filmcomprising vinylidene fluoride homopolymer which comprises the followingsteps (i), (ii) and (iii).

-   (i) A step for preparing a green powder product of vinylidene    fluoride homopolymer comprising crystal form I alone or as main    component by subjecting vinylidene fluoride to radical    polymerization in the presence of a radical polymerization    initiator.-   (ii) A step for forming a thin film on a substrate surface by using    vinylidene fluoride homopolymer which comprises crystal form I alone    or as main component and is obtained from the green powder product    of vinylidene fluoride homopolymer comprising crystal form I alone    or as main component.-   (iii) A step for subjecting the thin film of vinylidene fluoride    homopolymer formed in the above step (ii) to polarization treatment.

The method of the present invention is preferred because the vinylidenefluoride homopolymer can be applied to not only specific substrates suchas KBr and KCl but also any other substrates and also because coatingcan be easily carried out under usual coating conditions even withoutsetting any special coating conditions such as very low temperatures.

As a result, the thin film obtained in the step (ii) of the presentinvention comprises vinylidene fluoride homopolymer of crystal form I,and possesses capability of exhibiting ferroelectricity when subjectedto polarization treatment or the like.

The vinylidene fluoride homopolymer which is used for forming aferroelectric thin film of the present invention comprises crystal formI alone or as main component. When attention is given particularly tothe respective vinylidene fluoride homopolymers having crystal form I,II or III, it is preferable that the vinylidene fluoride homopolymershaving crystal form I are present in a ratio higher than those of thevinylidene fluoride homopolymers having crystal form II and vinylidenefluoride homopolymers having crystal form III.

The crystal form I of vinylidene fluoride homopolymer is characterizedin that a fluorine atom bonded to one carbon atom of the trunk chain inthe polymer molecule and a hydrogen atom bonded to the neighboringcarbon atom take a trans conformation (TT conformation), namely, thefluorine atom and hydrogen atom bonded to the neighboring carbon atomsare positioned oppositely at an angle of 180° when viewing from thecarbon-carbon bond.

In the present invention, the vinylidene fluoride homopolymer havingcrystal form I may take the TT conformation in the whole of one polymermolecule or in a part of the polymer molecule, and has the molecularchain of the TT conformation in at least four continuous vinylidenefluoride monomer units. In any cases, the carbon-carbon bond, in whichthe TT conformation constitutes the TT trunk chain, has a planar zigzagstructure, and the dipole moments of C—F₂ and C—H₂ bonds have moietiesin the vertical direction to the molecular chain. When the vinylidenefluoride homopolymer having crystal form I is subjected to IR analysis,there are characteristic peaks (characteristic absorptions) around 1,274cm⁻¹, 1,163 cm⁻¹ and 840 cm⁻¹. In powder X-ray diffraction analysis,there is a characteristic peak around 2θ=21°.

In the IR analysis, when characteristic absorptions of crystal form Iare recognized but characteristic absorptions of crystal forms II andIII are not recognized substantially, the crystal structure is called“all-I-form crystal structure”.

The crystal form II of vinylidene fluoride homopolymer is characterizedin that to a fluorine atom (or hydrogen atom) bonded to one carbon atomof trunk chain in the polymer molecule, a hydrogen atom (or fluorineatom) bonded to one neighboring carbon atom takes a trans form, and ahydrogen atom (or fluorine atom) bonded to another (opposite)neighboring carbon atom takes a gauche form (positioned at an angle of60°), and there are two or more continuous chains of this conformation(TGT G conformation). The molecular chain is of TGT G type and thedipole moments of C—F₂ and C—H₂ bonds have respective moieties in bothof the vertical and horizontal directions to the molecular chain. Whenthe vinylidene fluoride homopolymer having crystal form II is subjectedto IR analysis, there are characteristic peaks (characteristicabsorptions) around 1,212 cm⁻¹, 1,183 cm⁻¹ and 762 cm⁻¹. In powder X-raydiffraction analysis, there are characteristic peaks around 2θ=17.7°,18.3°and 19.9°.

In the IR analysis, when characteristic absorptions of crystal form IIare recognized but characteristic absorptions of crystal forms I and IIIare not recognized substantially, the crystal structure is called“all-II-form crystal structure”.

The crystal form III of vinylidene fluoride homopolymer is characterizedby having a conformation (T₃GT₃ G conformation) comprising TTconformation and TG conformation alternately continuously. The molecularchain is of T₃GT₃ G type and the dipole moments of C—F₂ and C—H₂ bondshave respective moieties in both of vertical and horizontal directionsto the molecular chain. When the vinylidene fluoride homopolymer havingcrystal form III is subjected to IR analysis, there are characteristicpeaks (characteristic absorptions) around 1,235 cm⁻¹ and 811 cm⁻¹. Inpowder X-ray diffraction analysis, there is a characteristic peak around2θ=18°.

Usually the presence of crystal form III is recognized in the form of amixture with the crystal form I and/or the crystal form II.

In the present invention, “comprising crystal form I as main component”means preferably that the proportion of vinylidene fluoride homopolymershaving crystal form I satisfies both of the following (Equation 1) and(Equation 2).100≧I-form/(I-form+II-form)≧50% by weight  (Equation 1)100≧I-form/(I-form+III-form)≧50% by weight  (Equation 2)

The presence and proportions of vinylidene fluoride homopolymers havingcrystal form I, II or III can be analyzed by various methods such asX-ray diffraction method and IR analysis method. In the presentinvention, the content F(I) of crystal form I in the vinylidene fluoridehomopolymer is calculated from a peak height (absorbance A) ofcharacteristic absorption of each crystal structure in an IR analysischart by the following methods.(1) Calculation of content (% by weight, F(I)×100) of I-form in amixture of I-form and II-form(1-1) EquationLaw of Beer: A=εbCwherein A represents an absorbance, ε represents a molar extinctioncoefficient, b represents an optical path length, and C represents aconcentration. When an absorbance of characteristic absorption ofcrystal form I is assumed to be A^(I), an absorbance of characteristicabsorption of crystal form II is assumed to be A^(II), a molarextinction coefficient of crystal form I is assumed to be ε^(I), a molarextinction coefficient of crystal form II is assumed to be ε^(II), aconcentration of crystal form I is assumed to be C^(I) and aconcentration of crystal form II is assumed to be C^(II), the followingequation is obtained.A^(I)/A^(II)=(ε^(I)/ε^(II))×(C^(I)/C^(II))  (1a)

When a correction factor (ε^(I)/ε^(II)) of the molar extinctioncoefficient is assumed to be E^(I)/^(II), the content F(I)(=C^(I)/(C^(I)+C^(II))) of crystal form I is represented by thefollowing equation. $\begin{matrix}{{F(I)} = \frac{\frac{1}{E^{I/{II}}} \times \frac{A^{II}}{A^{I}}}{{1 + {\frac{1}{E^{I/{II}}} \times \frac{A^{II}}{A^{I}}}} = \frac{A^{I}}{{E^{I/{II}}A^{II}} + A^{I}}}} & \left( {2a} \right)\end{matrix}$

Therefore when the correction factor E^(I)/^(II) is decided, the contentF(I) of crystal form I can be calculated from a measured absorbanceA^(I) of characteristic absorption of crystal form I and a measuredabsorbance A^(II) of characteristic absorption of crystal form II.

-   (1-2) Method of deciding correction factor E^(I)/^(II)

A sample in which the content F(I) of crystal form I is known isprepared by mixing a sample of all-I-form crystal structure (FIG. 1) anda sample of all-II-form crystal structure (FIG. 2), and is subjected toIR analysis. Then absorbances (peak height) A^(I) and A^(II) of eachcharacteristic absorption are read from the obtained chart (FIG. 3).

Then the absorbances are substituted in Equation (3a) obtained fromEquation (2a): $\begin{matrix}{E^{I/{II}} = \frac{A^{I} \times \left( {1 - {F(I)}} \right)}{A^{II} \times {F(I)}}} & \left( {3a} \right)\end{matrix}$and the correction factor E^(I)/^(II) is obtained. By changing themixing ratio of the samples repeatedly, each correction factorE^(I)/^(II) is obtained, and an average value of 1.681 is obtained.

As a characteristic absorption of crystal form I, 840 cm⁻¹ is used(Reference bulletin: Bachmann et al., Journal of Applied Physics, Vol.50, No. 10 (1979)), and 763 cm⁻¹ referred to in the mentioned bulletinis used as a characteristic absorption of crystal form II.

-   (2) Content F(I) of I-form in a mixture of I-form and III-form

Since a substance consisting of crystal form III is difficult to obtain,a mixture of II-form and III-form is used as a standard substance.

-   (2-1) Firstly, in the mentioned equation (2a), A^(I) and A^(II) are    assumed to be A¹l and A^(III), respectively and the correction    factor E^(II)/^(III) of the mixture of II-form and III-form is    assumed to be 0.81 from the bulletin (S. Osaki et al., Journal of    Polymer Science: Polymer Physics Edition, Vol. 13, pp. 1071 to 1083    (1975). The content of crystal form III in the standard mixture of    II-form and III-form is calculated by substituting A^(II) and    A^(III), which are read from the IR chart (FIG. 4) of the standard    mixture of II-form and III-form, in the equation (F(III)=0.573). As    a characteristic absorption of crystal form III, 811 cm⁻¹ is used    (Reference bulletin: Bachmann et al., Journal of Applied Physics,    Vol. 50, No. 10 (1979)).-   (2-2) Next, the standard mixture of II-form and III-form in which    the content of III-form is known is mixed with a substance of    all-I-form crystal structure in a specific ratio to prepare a    mixture of I-form, II-form and III-form, in which the content of    I-form is known. This mixture is subjected to IR analysis and A^(I)    and A^(III) are read from the chart (FIG. 5) and the correction    factor E^(I)/^(III)(ε^(I)/ε^(III)) is calculated from the mentioned    equation (3a) (A^(II) is changed to A^(III)). By changing the mixing    ratio of the standard mixture of II-form and III-form and the    substance of I-form repeatedly, each correction factor E^(I)/^(III)    is obtained, and an average value of 6.758 is obtained.-   (2-3) By using this correction factor E^(I)/^(III)=6.758, the    content F(I) of I-form in the mixture of I-form and III-form is    obtained from the mentioned equation (2a) (A^(II) is changed to    A^(III)).

Preferred vinylidene fluoride homopolymers used in the method of forminga thin film of the present invention are those satisfying both of thefollowing equations:100≧I-form/(I-form+II-form)≧60% by weightand100≧I-form/(I-form+III-form)≧60% by weightand more preferably those satisfying both of (Equation 3) and (Equation4).100≧I-form/(I-form+II-form)≧70% by weight  (Equation 3)100≧I-form/(I-form+III-form)≧70% by weight  (Equation 4).

Further preferred are those satisfying both of the following equations:100≧I-form/(I-form+II-form)≧80% by weightand100≧I-form/(I-form+III-form)≧80% by weight.Those vinylidene fluoride homopolymers are preferred since highferroelectricity can be exhibited by polarization treatment.

Further the proportion of crystal forms I satisfies preferably theequation:100≧I-form/(I-form+II-form+III-form)≧50% by weight,more preferably the equation:100≧I-form/(I-form+II-form+III-form)≧70% by weight,particularly preferably the equation:100≧I-form/(I-form+II-form+III-form)≧80% by weight.

A big feature of the present invention is that as a result of intensivestudies on polymerization method, the vinylidene fluoride homopolymercomprising crystal form I alone or as main component so as to satisfythe above-mentioned equations could be prepared in the form of greenpowder product after the polymerization even without a specificpost-treatment.

First, the step (i) is explained below.

The step (i) in the method of forming a ferroelectric thin film of thepresent invention is a step for preparing a green powder product ofvinylidene fluoride homopolymer comprising crystal form I alone or asmain component by subjecting vinylidene fluoride to radicalpolymerization in the presence of a radical polymerization initiator.

In the step (i) of the present invention, preparation of vinylidenefluoride homopolymer is carried out by subjecting vinylidene fluoride toradical polymerization in the presence of a radical polymerizationinitiator.

In addition to the radical polymerization initiator used in the presentinvention, there can be used light, heat or the like as a radicalgenerating source. When the preparation is carried out in the presenceof the radical polymerization initiator, a degree of polymerization canbe controlled, reaction can be advanced smoothly and a polymer can beobtained at high yield.

There can be used peroxides, azo initiators and the like as the radicalpolymerization initiator.

Examples of peroxides are, for instance, peroxydicarbonates such asn-propylperoxy dicarbonate, i-propylperoxy dicarbonate, n-butylperoxydicarbonate, t-butylperoxy dicarbonate andbis(4-t-butylcyclohexyl)peroxy dicarbonate; oxyperesters such asα,α′-bis(neodecanoylperoxy)diisopropylbenzene, cumylperoxyneodecanoate,1,1,3,3-tetramethylbutyl peroxyneodecanoate, 1-cyclohexyl-1-methylethylperoxyneodecanoate, t-hexyl peroxyneodecanoate, t-butylperoxyneodecanoate, t-hexyl peroxypivalate, t-butyl peroxypivalate,1,1,3,3-tetramethylbutylperoxy-2-ethylhexanoate,2,5-dimethyl-2,5-bis(2-ethylhexanoylperoxy)hexane,t-hexylperoxy-2-ethylhexanoate, t-butylperoxy-2-ethylhexanoate,t-butylperoxy isobutyrate, t-hexylperoxy isopropyl monocarbonate,t-butylperoxy maleic acid, t-butylperoxy-3,5,5-trimethylhexanoate,t-butylperoxy laurate, 2,5-dimethyl-2,5-bis(m-toluoylperoxy)hexane,t-butylperoxy isopropyl monocarbonate, t-butylperoxy-2-ethylhexylmonocarbonate, t-hexylperoxy benzoate,2,5-dimethyl-2,5-bis(benzoyl)hexane, t-butyl peroxyacetate, a mixture oft-butylperoxy-m-tolurate and peroxy benzoate, t-butylperoxy benzoate anddi-t-butylperoxy isophthalate; diacyl peroxides such as isobutylperoxide, 3,5,5-trimethylhexanoyl peroxide, octanoyl peroxide, lauroylperoxide, stearoyl peroxide, succinic acid peroxide, m-toluoyl peroxideand benzoyl peroxide; peroxy ketals such as1,1-bis(t-hexylperoxy)-3,3,5-trimethylcyclohexane,1,1-bis(t-hexylperoxy)cyclohexane,1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane,1,1-bis(t-butylperoxy)-2-methylcyclohexane,1,1-bis(t-butylperoxy)cyclohexane, 2,2-bis(t-butylperoxy)butane,n-butyl-4,4-bis(t-butylperoxy)valerate and2,2-bis(4,4-di-t-butylperoxycyclohexyl)propane; dialkyl peroxides suchas α,α′-bis(t-butylperoxy)diisopropylbenzene, dicumyl peroxide,2,5-dimethyl-2,5bis(t-butylperoxy)hexane, t-butylcumyl peroxide,di-t-butyl peroxide and 2,5-dimethyl-2,5bis(t-butylperoxy)hexyne-3;hydroperoxides such as p-menthane hydroperoxide, diisopropylbenzenehydroperoxide, 1,1,3,3-tetramethylbutyl hydroperoxide, cumenehydroperoxide and t-butyl hydroperoxide; persulfates such as ammoniumpersulfate, potassium persulfate and sodium persulfate; perchloricacids, hydrogen peroxides and the like.

Also there can be used peroxides having fluorine atom. Preferredexamples thereof are one or two or more of fluorine-containing diacylperoxides, fluorine-containing peroxy dicarbonates, fluorine-containingperoxy diesters and fluorine-containing dialkyl peroxides. Among them,preferred are difluoroacyl peroxides such as pentafluoropropionoylperoxide (CF₃CF₂COO)₂, heptafluorobutyryl peroxide (CF₃CF₂CF₂COO)₂,7H-dodecafluoroheptanoyl peroxide (CHF₂CF₂CF₂CF₂CF₂CF₂COO)₂ and thelike.

Examples of azo type radical polymerization initiator are, for instance,2,2′-azobisisobutyronitrile, 2,2′-azobis(2,4-dimethylvaleronitrile),2,2′-azobis(2-methylvaleronitrile),2,2′-azobis(2-cyclopropylpropionitrile), dimethyl2,2′-azobis(isobutyrate), 2,2′-azobis[2-(hydroxymethyl)propionitrile]and 4,4′-azobis(4-cyanopentenic acid).

Among the radical polymerization initiators, particularly preferred areperoxy dicarbonates, difluoroacyl peroxides, oxyperesters, persulfatesand the like.

With respect to the amount of radical polymerization initiator, a lowerlimit thereof to 1 mole of vinylidene fluoride monomer is 0.0001 mole,preferably 0.01 mole, more preferably 0.03 mole, particularly preferably0.05 mole, and an upper limit thereof to 1 mole of vinylidene fluoridemonomer is 0.9 mole, preferably 0.5 mole, more preferably 0.1 mole,particularly preferably 0.08 mole. Too small amount of radicalpolymerization initiator is not preferred because polymerizationreaction is difficult to be advanced, and too large amount thereof isnot preferred because it is difficult to obtain a crystalline polymer.

It is preferable that the radical polymerization is carried out also inthe presence of a chain transfer agent (telogen) since the proportion ofcrystal form I is increased. There can be used known chain transferagents. For example, various chlorine compounds, bromine compounds,iodine compounds and alcohols may be used. There can be usedparticularly preferably bromine compounds or iodine compounds having 1to 20 carbon atoms which contain at least one moiety represented by theformula (1):

wherein X¹ is iodine atom or bromine atom; Rf¹ and Rf² are the same ordifferent and each is selected from fluorine atom or perfluoroalkylgroups having 1 to 5 carbon atoms. Particularly preferred are brominecompounds or iodine compounds having the moiety represented by theformula (1).

When the bromine compound or iodine compound having the moietyrepresented by the formula (1) is used as a chain transfer agent(telogen) for the polymerization, a polymer having a narrow molecularweight distribution and a polymer chain having a low branch ratio can besynthesized, and a vinylidene fluoride homopolymer, in which the contentof crystal form I is high, can be obtained.

Examples of the moiety represented by the formula (1) are:

and the like. Particularly preferred are iodine compounds since amolecular weight distribution can be made narrower, and as a result, agreen powder product of vinylidene fluoride homopolymer, in which thecontent of crystal form I is high, can be obtained.

Also it is preferable that in the moiety of the formula (1), Rf¹ and Rf²are F since a vinylidene fluoride homopolymer, in which the content ofcrystal form I is high, can be obtained.

Among the bromine compounds or iodine compounds having the moietyrepresented by the formula (1), preferred are polyfluoro compoundshaving the moiety of the formula (1), more preferably perfluorocompounds having the moiety of the formula (1) since polymerizationreaction advances at higher yield and a polymer having a narrowmolecular weight distribution and fewer branched chains can be obtained.

Particularly preferred is at least one of perfluoro iodides representedby the formula (2):X²-(CF₂)_(n)-I  (2)wherein X² is fluorine atom or iodine atom, n is an integer of 1 to 20,or perfluoro bromides obtained by replacing the iodine atom in the aboveformula (2) by bromine atom.

Examples of the perfluoro compounds are, for instance, iodine compoundssuch as perfluoro monoiodide compounds such as monoiodideperfluoromethane, monoiodide perfluoroethane, monoiodideperfluoropropane, monoiodide perfluorobutane (for example, 2-iodideperfluorobutane, 1-iodide perfluoro(1,1-dimethylethane)), monoiodideperfluoropentane (for example, 1-iodide perfluoro(4-methylbutane)),1-iodide perfluoro-n-nonane, monoiodide perfluorocyclobutane, 2-iodideperfluoro(1-cyclobutyl)ethane and monoiodide perfluorocyclohexane;perfluoro diiodide compounds such as diiodide perfluoromethane,1,2-diiodide perfluoroethane, 1,3-diiodide perfluoro-n-propane,1,4-diiodide perfluoro-n-butane, 1,7-diiodide perfluoro-n-octane,1,2-di(iodidedifluoromethyl) perfluorocyclobutane and 2-iodide1,1,1-trifluoroethane, and bromine compounds obtained by replacing theiodine atoms of those iodine compounds by bromine atoms.

More preferably the preparation of green powder product of vinylidenefluoride homopolymer comprising crystal form I alone or as maincomponent in the step (i) is characterized in that the radicalpolymerization is carried out in the presence of a radicalpolymerization initiator and at least one of perfluoro iodidesrepresented by the formula (2):X²-(CF₂)_(n)-I  (2)wherein X² is a fluorine atom or iodine atom, n is an integer of 1 to20, and a number average degree of polymerization of vinylidene fluorideunits in the polymer is adjusted to 4 to 20, preferably 4 to 15, therebyassuring the preparation of the homopolymer.

Namely, it is important to use an iodine compound having a linearfluoroalkyl group, which makes it easier to prepare a polymer having ahigh purity of crystal form I as compared with use of a branchedfluoroalkyl group such as (CF₃)₂CF-I.

In the iodine compounds of the formula (2), it is more preferable that nis 1 or 4 m, in which m is 1 to 5.

Examples of the iodine compounds of the formula (2) are, for instance,CF₃I, F(CF₂)₄I, F(CF₂)₈I and in addition, perfluoro diiodidesrepresented by I(CF₂CF₂)_(n1)I, in which n1 is an integer of 1 to 5 [forexample, I(CF₂CF₂)I, I(CF₂CF₂)₂I, I(CF₂CF₂)₃I, I(CF₂CF₂)₄I and like].Particularly preferred are CF₃I and I(CF₂CF₂)_(n1)I, in which n1 is aninteger of 1 to 5, and among them, CF₃I and I(CF₂CF₂)₂I are preferred.

When those iodine compounds are used as a chain transfer agent(telogen), the green powder product of vinylidene fluoride homopolymercomprising crystal form I can be obtained at high purity in highefficiency.

With respect to the amount of iodine compound, a lower limit thereof to1 mole of vinylidene fluoride monomer is 0.01 mole, preferably 0.02mole, more preferably 0.03 mole, particularly preferably 0.08 mole, andan upper limit thereof to 1 mole of vinylidene fluoride monomer is 10mole, preferably 6 mole, more preferably 2 mole, particularly preferably1 mole.

If the amount of iodine compound is too small, a degree ofpolymerization is increased excessively and as a result, the content ofcrystal form I tends to be decreased, and if the amount of iodinecompound is too large, there is a tendency that polymerization reactionis difficult to be advanced, yield is lowered and a degree ofpolymerization is decreased excessively.

With respect to the amount of radical polymerization initiator when theiodine compound is used, a lower limit thereof to 1 mole of iodinecompound is 0.0001 mole, preferably 0.01 mole, more preferably 0.03mole, particularly preferably 0.04 mole, and an upper limit thereof to 1mole of iodine compound is 0.9 mole, preferably 0.5 mole, morepreferably 0.1 mole, particularly preferably 0.08 mole.

When attention is given to recurring units of only vinylidene fluoridein the vinylidene fluoride homopolymer, a lower limit of number averagedegree of polymerization thereof is preferably 4, particularlypreferably 5, and an upper limit thereof is preferably 20, morepreferably 15, further preferably 12, particularly preferably 10. Toohigh number average degree of polymerization is not preferred because aratio of crystal form I is decreased.

In the process for preparing vinylidene fluoride homopolymer of thepresent invention, there can be used a method of bulk polymerizationwithout using a polymerization solvent, a method of solutionpolymerization using a solvent for dissolving monomers in apolymerization system, a method of suspension polymerization using asolvent for dissolving and dispersing monomers in a polymerizationsystem and as case demands, a dispersion medium such as water, a methodof emulsion polymerization in an aqueous solvent containing anemulsifying agent and the like.

Among them, solution polymerization and suspension polymerization arepreferred since the degree of polymerization is easily controlled.

Examples of the polymerization solvents which can be used for solutionpolymerization and suspension polymerization are ketone solvents such asacetone, methyl ethyl ketone and methyl isobutyl ketone; ester solventssuch as ethyl acetate, cellosolve acetate, n-butyl acetate, isobutylacetate, methyl cellosolve acetate and carbitol acetate; alcoholsolvents such as methyl alcohol, ethyl alcohol, isopropyl alcohol,n-butyl alcohol, isobutyl alcohol, tert-butyl alcohol, sec-butylalcohol, tert-amyl alcohol, 3-pentanol, octyl alcohol and3-methyl-3-methoxybutanol; aromatic solvents such as benzene, tolueneand xylene; and the like. Also there can be used fluorine-containingsolvents such as CHF₂CF₂OCHF₂, (CF₃)₂CFOCH₃, CF₃CF₂CF₂OCH₃, CHF₂CF₂OCH₃,CF₃CF₂CH₂OCHF₂, CF₃CFHCF₂OCH₃, CHF₂CF₂OCH₂CF₃, CF₃CF₂CF₂CF₂OCH₃,CF₃CF₂CH₂OCF₂CHF₂, (CF₃)₂CHCF₂OCH₃, CF₃CFHCF₂OCH₂CF₃,CF₃CF₂CF₂CF₂OCH₂CH₃, CF₃CHFCF₂OCH₂CF₂CF₃, CF₃CHFCF₂CH₂OCHF₂,CHF₂CF₂CH₂OCF₂CHF₂, CF₃CFHCF₂OCH₂CF₂CF₂H, CHF₂CF₂CF₂CF₂CH₂OCH₃, C₆F₁₂,C₉F₁₈, C₆F₁₄, CF₃CH₂CF₂CH₃, CHF₂CF₂CF₂CHF₂, (CF₃)₂CFCHFCHFCF₃,CF₃CHFCHFCF₂CF₃, (CF₃)₂CHCF₂CF₂CF₃, C₄H₂F₆, CF₃CF₂CHF₂, CF₂ClCF₂CF₂CHF₂,CF₃CFClCFClCF₃, CF₂ClCF₂CF₂CF₂Cl, CF₂ClCF₂CF₂CF₂CF₂CF₂CHF₂,CF₂ClCFClCFClCF₂Cl, HCFC-225, HCFC-141b, CF₂ClCF₂Cl, CF₂ClCFCl₂,H(CF₂)_(n)H (n is an integer of 1 to 20), CF₃O(C₂F₄O)_(n)CF₂CF₃ (n is 0or an integer of 1 to 10) and N(C₄F₉)₃.

Particularly preferred are fluorine-containing solvents because a degreeof polymerization is easily controlled. Among them, particularlypreferred are fluorine-containing solvents such as HCFC-225, HCFC-141b,CF₂ClCFClCFClCF₂Cl, CF₂ClCF₂Cl, CF₂ClCFCl₂, H(CF₂)_(n)H (n is an integerof 1 to 20) and CF₃O(C₂F₄O)_(n)CF₂CF₃ (n is 0 or an integer of 1 to 10)and N(C₄F₉)₃.

A polymerization temperature can be optionally selected depending onkind of radical polymerization initiator, and is usually from −10° C. to200° C. A lower limit thereof is preferably 5° C., more preferably 10°C. and an upper limit thereof is preferably 150° C., more preferably100° C.

The vinylidene fluoride homopolymer obtained in the step (i) of thepresent invention becomes a vinylidene fluoride homopolymer havingparticularly high ratio of crystal form I by using the specific chaintransfer agent (for example, the chain transfer agent of the formula(2)) mentioned above, but is not limited thereto.

Namely, the vinylidene fluoride homopolymer obtained in the step (i) is,for example, a vinylidene fluoride homopolymer represented by theformula (4):Y-(A¹)-X³  (4)wherein A¹ is a structural unit of vinylidene fluoride homopolymerhaving a number average degree of polymerization of 5 to 12, X³ isiodine atom or bromine atom, Y is a residue of chain transfer agent.

This vinylidene fluoride homopolymer has a residue (for example, CF₃group) of the chain transfer agent at one end of one polymer moleculeand an iodine atom at another end thereof. The structural unit A¹ has arecurring unit of vinylidene fluoride having a number average degree ofpolymerization of 5 to 12.

This polymer of the formula (4) has a particularly high purity ofcrystal form I when the chain transfer agent of the formula (2)mentioned above is used.

If the number average degree of polymerization of the structural unit A¹is not more than 4, crystals become difficult to be formed at roomtemperature, and if the number average degree of polymerization is notless than 13, a purity of crystal form I is decreased (for example, aratio of crystal form II is increased).

Also it is particularly preferable that one end of the polymer moleculeis CF₃ group because a purity of crystal form I is increased. Forexample, when one end is a long chain perfluoroalkyl group or a branchedperfluoroalkyl group, a purity of crystal form I is decreased (forexample, a ratio of crystal form II is increased).

A molecular weight distribution of the polymer of the formula (4) variesdepending on the average degree of polymerization. For example, Mw/Mnobtained by GPC analysis is not less than 1 and not more than 3,preferably not more than 2, more preferably not more than 1.5. If themolecular weight distribution is increased, a purity of crystal form Itends to be decreased.

The polymer of the formula (4) may be constructed only by polymermolecules of the formula (4-1):Y—(CH₂CF₂)_(n)-X³  (4-1)in which the vinylidene fluoride units face toward the same direction inone polymer molecule, or may comprise polymer molecules having thestructure of the formula (4-2):Y—(CH₂CF₂)_(n1)(CF₂CH₂)_(n2)-X³  (4-2)in which a part of the vinylidene fluoride units are bonded toward theopposite directions in one polymer molecule, wherein Y and X³ are asdefined in the formula (4), n1+n2=n=1 to 20.

Particularly preferred is the polymer consisting of polymer molecules ofthe formula (4-1), in which the vinylidene fluoride units face towardthe same direction.

Even in the case of a mixture of polymer molecules of the formulae (4-1)and (4-2), the lower the n2 ratio (called abnormal bonding ratio (ratioof head to head and tail to tail additions)), the more preferable. Forexample, preferred is a mixture having an abnormal bonding ratio:(n2/(n+n1+n2))×100 of not more than 20%, further not more than 10%,particularly not more than 5%, which can be calculated by NMR analysis.

Another vinylidene fluoride homopolymer obtained in the step (i) of thepresent invention is, for example, a vinylidene fluoride homopolymerrepresented by the formula (5):X⁴-(A²)-(Rf³)_(m)-(A³)-X⁵  (5)wherein m is an integer of 1 to 5; A² and A³ are the same or differentand each is a structural unit of vinylidene fluoride homopolymer and thesum of number average degree of polymerization of the structural unitsA² and A³ is 2 to 20; Rf³ is a perfluoroalkylene group such as CF₂CF₂;X⁴ and X⁵ are the same or different and each is iodine atom or bromineatom. When Rf³ is CF₂CF₂ and both of X⁴ and X⁵ are iodine atoms,unexpectedly the polymer has a high purity of crystal form I.

The sum of number average degree of polymerization of the structuralunits A² and A³ is selected within a range of 2 to 20, and a lower limitthereof is more preferably 4, further preferably 5, and an upper limitthereof is preferably 15, further preferably 12.

Namely, if the number average degree of polymerization is too low,crystals are difficult to be formed at room temperature, and if thenumber average degree of polymerization is too high, a purity of crystalform I is decreased (for example, a ratio of crystal form II isincreased).

In the polymer of the formula (5), m can be selected within a range of 1to 5 and is more preferably 2, in which a purity of crystal form I isparticularly high.

The polymer of the formula (5) can be synthesized by various processes,for example, by the above-mentioned process by using a chain transferagent of the formula (5-1):X⁴-(Rf³)_(m)-X⁵  (5-1)wherein m is an integer of 1 to 5, Rf³, X⁴ and X⁵ are as defined above.The use of this chain transfer agent is preferred because a polymerhaving a narrow molecular weight distribution can be synthesized,thereby enabling the purity of crystal form I to be increased.

In the polymer of the formula (5), a molecular weight distribution ofthe structural units A² and A³ varies depending on the sum of numberaverage degree of polymerization of the structural units A² and A³. Forexample, Mw/Mn obtained by GPC analysis is not less than 1 and not morethan 3, preferably not more than 2, more preferably not more than 1.5.If the molecular weight distribution is increased, a purity of crystalform I tends to be decreased.

The vinylidene fluoride homopolymers of the formulae (4) and (5) arepreferably those having crystal form I satisfying (Equation 1) and(Equation 2), further preferably those having, at a high purity, crystalform I satisfying (Equation 3) and (Equation 4), thereby enablingferroelectric characteristics to be imparted effectively to the thinfilm of the present invention.

In the method of forming a ferroelectric thin film of the presentinvention, also there can be used end-modified vinylidene fluoridehomopolymers obtained by modifying the end groups X³, X⁴ and X⁵ (all areiodine atoms or bromine atoms) of the vinylidene fluoride homopolymersof the formula (4) (including the formulae (4-1) and (4-2)) and theformula (5) obtained in the step (i) of the present invention to H or F(n=0) or an alkyl group (n=1 to 4) which may have fluorine atom, whichis represented by the formula (6):—C_(n)X⁶ _(2n+1)  (6)wherein n is 0 or an integer of 1 to 4; X⁶ is H or F.

When modifying the end groups X³, X⁴ and X⁵ (all are iodine atoms orbromine atoms) to the end group of the formula (6), iodine atom orbromine atom may be modified directly to H, F or the alkyl group or maybe once modified to other functional group and then modified to the endgroup of the formula (6).

In the end group of the formula (6), it is particularly preferable thatn is zero, namely the end group is H or F because ferroelectricity isincreased when n is smaller.

In the method of forming a ferroelectric thin film of the presentinvention, then the step (ii) for forming a thin film on a substrate iscarried out by using the vinylidene fluoride homopolymer comprisingcrystal form I alone or as main component which is obtained in thementioned step (i).

In the step (ii), the green powder product of vinylidene fluoridehomopolymer prepared in the step (i) may be applied directly on asubstrate, or the vinylidene fluoride homopolymer which comprisescrystal form I alone or as main component and is obtained by subjectingthe green powder product of vinylidene fluoride homopolymer prepared inthe step (i) to any treatments within a range not having adverse effecton crystal form I may be applied on a substrate.

Examples of steps for such treatments are, for instance, a washing stepwhich is carried out just after the step (i) for removing low molecularweight impurities in the green polymer powder, a step for separating thevinylidene fluoride homopolymers comprising crystal form I alone or asmain component and having a specific molecular weight, steps forre-precipitation and re-crystallization, a heating step for drying, avacuum treatment step, a heat-treatment step for crystal growth and thelike.

Among those steps, by separating the homopolymers having a specificmolecular weight by the separation step, a purity of I-form crystal isincreased, thereby enabling ferroelectric characteristics to be impartedmore effectively to the ferroelectric thin film of the presentinvention. The separation step can be carried out preferably, forexample, by re-precipitation method, distillation method, chromatographymethod, vapor deposition method or the like method.

According to the re-precipitation method, vinylidene fluoridehomopolymers having the same molecular weight can be separated byallowing the green powder product of vinylidene fluoride homopolymer tobe dissolved in as small an amount as possible of solvent (good solvent)and then pouring into a solvent (poor solvent), in which the greenpowder product of vinylidene fluoride homopolymer is low in solubility,for re-precipitation of the vinylidene fluoride homopolymer.

In this case, it is preferable that the green powder product ofvinylidene fluoride homopolymer is dissolved in an amount of 1 to 80% byweight, preferably 1 to 70% by weight, more preferably 1 to 50% byweight to the good solvent. Also it is preferable that the amount ofpoor solvent is about 10 to about 20 times the amount of good solvent. Are-precipitation temperature is usually −30° C. to 150° C., preferably0° C. to 80° C., more preferably 25° C. to 50° C.

The above-mentioned good solvent and poor solvent may be optionallyselected depending on solubility of vinylidene fluoride homopolymer tobe re-precipitated. There can be used preferably, for example, ketonesolvents such as acetone, methyl ethyl ketone, methyl isobutyl ketone,cyclohexanone and acetyl acetone; ester solvents such as ethyl acetate,cellosolve acetate, n-butyl acetate, isobutyl acetate, methyl cellosoveacetate, carbitol acetate and dibutyl phthalate; aldehyde solvents suchas benzaldehyde; amine solvents such as dimethylamine, dibutylamine,dimethylaniline, methylamine and benzylamine; amide solvents such asN,N-dimethylformamide, N,N-dimethylacetamide and N-methyl-2-pyrrolidone;carboxylic acid anhydride solvents such as acetic anhydride; carboxylicacid solvents such as acetic acid; halogen solvents such as chloroform,dichloromethane, 1,2-dichloroethane, chlorobenzene, benzyl chloride and1,1,2,2-tetrachloroethane; ether solvents such as tetrahydrofuran anddioxane; sulfone amide solvents such as dimethyl sulfoxide; aliphatichydrocarbon solvents such as hexane, heptane, octane and petroleumether; alcohol solvents such as methanol, ethanol and 1-propanol;aromatic hydrocarbon solvents such as benzene, toluene, xylene andstyrene; and solvent mixtures of two or more thereof.

According to the distillation method, vinylidene fluoride homopolymershaving the same molecular weight can be efficiently separated bydistilling the green powder product of vinylidene fluoride homopolymerunder a specific pressure (reduced pressure) and a specific temperature.

A distilling pressure is usually 0.1 Pa to 101 KPa, preferably 1 Pa to50 KPa, more preferably 100 Pa to 1 KPa. A distilling temperature isusually 0° C. to 500° C., preferably 0° C. to 250° C., more preferably25° C. to 200° C.

According to the washing method, vinylidene fluoride homopolymers havingthe same molecular weight can be separated by subjecting the greenpowder product of vinylidene fluoride homopolymer to washing with asolvent.

The solvent used for the washing may be optionally selected from thosebeing capable of dissolving the vinylidene fluoride homopolymer.Concretely the same solvents as exemplified in the re-precipitationmethod can be used.

A solvent temperature at washing is usually −30° C. to 150° C.,preferably 0° C. to 80° C., more preferably 25° C. to 50° C.

The number of washing cycles varies depending on kind of solvent for thewashing. In principle, the washing may be carried out optional times,usually not more than 100 times, preferably not more than 50 times, morepreferably not more than 10 times.

According to the chromatography method, vinylidene fluoride homopolymershaving the same molecular weight can be separated efficiently.

When the mobile phase is one dissolving vinylidene fluoride homopolymer,any of known methods may be employed. For example, liquid phasechromatography and gas chromatography are used preferably. A temperaturein the chromatography method is usually −30° C. to 150° C., preferably0° C. to 100° C., more preferably 25° C. to 80° C.

According to the vapor deposition method, vinylidene fluoridehomopolymers having the same molecular weight can be efficientlyseparated by vapor deposition of the green powder product of vinylidenefluoride homopolymer under a specific pressure (reduced pressure) and aspecific temperature.

In the vapor deposition, the green powder product of vinylidene fluoridehomopolymer is subjected to heating or cooling, and a vapor depositiontemperature is usually −30° C. to 1,000° C., preferably 0° C. to 800°C., more preferably 0° C. to 500° C. A vapor deposition pressure in asystem is usually 1×10⁻⁶ Pa to 100 KPa, preferably not more than 1 KPa,more preferably not more than 1 Pa.

It is preferable to employ the distillation method or chromatographymethod because vinylidene fluoride homopolymers having the samemolecular weight can be separated easily efficiently.

As the molecular weight distribution is made narrower by such separationsteps, a purity of crystal form I is increased and ferroelectriccharacteristics can be imparted more effectively to the ferroelectricthin film of the present invention. Therefore it is preferable toincrease the purity of vinylidene fluoride homopolymers having the samemolecular weight to not less than 70% by weight, further not less than80% by weight, further preferably not less than 90% by weight,particularly not less than 95% by weight.

Also the step (ii) for forming a thin film may be carried out after astep of blending a solvent and additives to the vinylidene fluoridehomopolymer comprising crystal form I alone or as main component andforming into a coating.

In the step (ii) of the present invention, there can be used variousmethods of forming a thin film. There can be preferably used, forexample, a method (coating solution method) of dissolving or dispersingvinylidene fluoride homopolymer in a liquid medium and applying in theform of a coating solution (coating composition); a method (powdercoating method) of applying vinylidene fluoride homopolymer in the formof powder directly on a substrate; a method (vacuum vapor depositionmethod) of subjecting vinylidene fluoride homopolymer powder tosublimation in vacuo and/or under heating and then coating bydeposition, and the like method.

In the method of applying the vinylidene fluoride homopolymer in theform of a coating solution (coating composition), there can be usedliquid medium which can dissolve or uniformly disperse the vinylidenefluoride homopolymer. In order to control a thickness of the thin film,particularly preferred are liquid medium which can dissolve thevinylidene fluoride homopolymer.

Preferred examples of the liquid medium are, for instance, ketonesolvents such as acetone, methyl ethyl ketone, methyl isobutyl ketone,cyclohexanone and acetyl acetone; ester solvents such as ethyl acetate,cellosolve acetate, n-butyl acetate, isobutyl acetate, methyl cellosolveacetate, carbitol acetate and dibutylphthalate; aldehyde solvents suchas benzaldehyde; amine solvents such as dimethylamine, dibutylamine,dimethylaniline, methylamine and benzylamine; amide solvents such asN,N-dimethylformamide, N,N-dimethylacetamide and N-methyl-2-pyrrolidone;carboxylic acid anhydride solvents such as acetic anhydride; carboxylicacid solvents such as acetic acid; halogen solvents such as chloroform,dichloromethane, 1,2-dichloroethane and 1,1,2,2-tetrachloroethane; ethersolvents such as tetrahydrofuran and dioxane; sulfone amide solventssuch as dimethyl sulfoxide; and the like.

Particularly preferred are ketone solvents and amide solvents becausevinylidene fluoride homopolymer is dissolved therein satisfactorily.

Also when vinylidene fluoride homopolymer is uniformly dispersed stablyin the form of fine particles in a medium, a thin film can be formedeven in the case of the homopolymer being insoluble in a liquid solvent.For example, an aqueous dispersion of vinylidene fluoride homopolymercan be used.

A concentration of vinylidene fluoride homopolymer in the coatingsolution varies depending on an intended coating thickness, a viscosityof the coating solution, etc. The concentration is not less than 0.1% byweight, preferably not less than 0.5% by weight, more preferably notless than 1% by weight, and not more than 50% by weight, preferably notmore than 30% by weight, more preferably not more than 20% by weight.

For applying the coating solution on a substrate, there can be usedknown coating methods such as spin coating, dip coating, spray coating,roll coating and gravure coating. For efficiently forming a thin film,the spin coating method and gravure coating method are preferred, andparticularly the spin coating method is preferred.

After the application by the above-mentioned method, a drying step maybe carried out for removing the solvent. For the drying, for example,air drying at room temperature, drying by heating, vacuum drying and thelike can be used. In the drying, attention should be paid not to dryexcessively at high temperature since there is a case where the crystalform I is changed.

Accordingly, it is preferable to dry by heating at a temperature lowerthan a melting point of vinylidene fluoride homopolymer. The temperaturefor drying by heating varies depending on a boiling point of a solventto be used, and is not less than 30° C., preferably not less than 40°C., more preferably not less than 50° C. and not more than 100° C.,preferably not more than 80° C., more preferably not more than 70° C.

The thus formed thin film of vinylidene fluoride homopolymer on asubstrate by application in the form of a coating solution maintainscrystal form I and has an ability of exhibiting excellentferroelectricity.

Also preferred is a method of forming a thin film on a substrate by thevacuum vapor deposition method by using a vacuum vapor depositionequipment.

A vacuum vapor deposition temperature and vacuum degree are optionallyselected depending on a degree of polymerization and sublimationproperty of vinylidene fluoride homopolymer. The deposition temperatureis from room temperature to 200° C., preferably not more than 100° C.The substrate temperature is from 0° C. to 100° C., preferably not lessthan room temperature and not more than 50° C. The vacuum degree is notmore than 10⁻² Pa, preferably not more than 10⁻⁴ Pa.

In this vacuum vapor deposition method, by use of the method of forminga thin film of the present invention including the step (i), a thin filmof vinylidene fluoride homopolymer having crystal form I can be formedeasily under normal conditions such as room temperature even withoutsetting the substrate particularly at very low temperature.

In the method of forming a ferroelectric thin film of the presentinvention, kind of a substrate is not limited, and a thin film ofvinylidene fluoride homopolymer having crystal form I can be formed onvarious kinds of substrates.

For the purpose of forming a ferroelectric thin film, preferred areelectrically conductive substrates being capable of forming anelectrode. Accordingly in addition to metallic substrates, insulatingsubstrates such as silicon substrates, glass substrates, ceramicsubstrates and resin substrates on which a thin film of electricallyconductive material is formed are also preferred as the electricallyconductive substrates.

As an electrically conductive substrate or a metallic material for thinfilm, there can be used aluminum, copper, chromium, nickel, zinc,stainless steel, gold, silver, platinum, tantalum, titanium, niobium,molybdenum, indium tin oxide (ITO) and the like. Particularly preferredare silicon wafers on which a thin film of aluminum, gold, silver,platinum, tantalum, titanium or the like is formed. As the metallicsubstrate, aluminum, copper, gold, silver and platinum are alsopreferred.

Those electrically conductive thin films provided on a substrate surfacemay be previously subjected to patterning of intended circuit by a knownmethod such as photolithography, mask deposition or the like, as casedemands.

Also a substrate which is coated with a polymer on its surface may beused.

On those substrates are formed thin films of vinylidene fluoridehomopolymer having crystal form I by the mentioned method (step (ii)).

A thickness of the thin film of vinylidene fluoride homopolymer havingcrystal form I is optionally selected depending on an object andapplication of intended laminated article. The thickness is usually notless than 1 nm, preferably not less than 5 nm, particularly preferablynot less than 10 nm, and not more than 10 μm, preferably not more than 1μm, particularly preferably not more than 500 nm.

In the method of forming a ferroelectric thin film of the presentinvention, the polarization step (iii) is further carried out afterforming the thin film according to the step (ii). The polarization step(iii) is carried out for the purpose of making the thin film of thepresent invention surely exhibit ferroelectricity.

For the polarization, known methods can be used similarly. For example,there can be used a method of carrying out deposition of an electrode onthe film or contacting an electrode to the film and then applyingelectric field of direct or alternating current or direct or alternatingvoltage on the electrode (poling treatment), a method of coronadischarging or the like method.

The applied electric field in the polarization step (iii) can beoptionally selected depending on the thickness of the thin film, aproportion of crystal form I, etc., and is usually not less than 10MV/m, preferably not less than 50 MV/m, more preferably not less than 80MV/m, and not more than dielectric breakdown electric voltage,preferably not more than 250 MV/m, more preferably not more than 200MV/m. If the applied electric field is too low or the applying time istoo short, enough polarization is not attained. Also too high appliedelectric field or too long applying time is not preferred becausebonding of polymer molecules is cleaved even partially.

The applying time is usually not less than 20 nanoseconds, preferablynot less than 1 second, more preferably not less than 1 minute, and upto about 48 hours, preferably six hours, more preferably two hours.

A thin film temperature in the polarization step (iii) is usually notless than 0° C., preferably not less than 10° C., more preferably notless than 25° C., and not more than a crystalline melting point ofvinylidene fluoride homopolymer, preferably not more than 120° C., morepreferably not more than 85° C.

In the method of forming a ferroelectric thin film of the presentinvention, after forming the thin film of vinylidene fluoridehomopolymer on a substrate according to the step (ii), a step for heattreating (heat treating step (iv)) may be carried out before thepolarization step (iii) or at the same time when the polarization step(iii) is carried out, for the purpose of enhancing ferroelectriccharacteristics of the formed thin film of vinylidene fluoridehomopolymer. The heat treating step (iv) of the thin film of vinylidenefluoride homopolymer is usually carried out for the purpose of growth ofcrystals in the thin film of vinylidene fluoride homopolymer to increasethe crystal size, and as a result, ferroelectric characteristics can beenhanced.

A heat treating temperature in the heat treating step (iv) can beoptionally selected depending on a number average degree ofpolymerization and crystalline melting point of the vinylidene fluoridehomopolymer and kind of a substrate, and is usually not less than 50°C., preferably not less than 60° C., more preferably not less than 70°C., particularly preferably not less than 80° C., and an upper limitthereof is usually a temperature lower than a crystalline melting pointof the vinylidene fluoride homopolymer, preferably a temperature lowerthan the crystalline melting point by 5° C., more preferably atemperature lower than the crystalline melting point by 10° C.

A heat treating time is usually not less than about 10 minutes,preferably not less than 20 minutes, more preferably not less than 30minutes, and not more than about 10 hours, preferably not more than 5hours, more preferably not more than 3 hours, particularly preferablynot more than about 2 hours. It is preferable that after the heating,the film is allowed to stand at room temperature for air cooling slowly.

It is preferable to use the preferred vinylidene fluoride homopolymer ofthe present invention comprising form I alone or as main componentbecause enough ferroelectric characteristics can be exhibited evenwithout carrying out the above-mentioned heat treating step (iv).

Further the thin film of vinylidene fluoride homopolymer in the thusobtained laminated article may be subjected to patterning of intendedcircuit by a known method such as photolithography, mask deposition orthe like, as case demands.

Also as case demands, a layer of other material may be provided on thethin film of vinylidene fluoride homopolymer in the thus obtainedlaminated article.

For example, it is possible to make multiple layers by providing thethin film of vinylidene fluoride homopolymer between electricallyconductive material layers being capable of becoming the same electrodeas mentioned above or insulating layers of silicon, ceramic, resin orthe like in the form of sandwich.

The thus obtained laminated article has ferroelectricity.

In the present invention, ferroelectricity is a property that permanentdipoles inside a substance are oriented in the same direction by actionof any force and the substance has polarization even when an electricfield is not applied (polarization generated even without an electricfield is called spontaneous polarization). Also ferroelectricity is aproperty that the spontaneous polarization can be inverted by an outsideelectric field. Whether or not a substance has ferroelectricity is knownby the fact that when examining a relation between the electric field Eand the electric displacement density D, if the substance is aferroelectric substance, a hysteresis curve like that of a ferromagneticsubstance is shown when an alternating electric field having a largeamplitude to a certain extent is applied thereto.

According to the method of the present invention, for example, withrespect to a laminated article comprising a layer of vinylidene fluoridehomopolymer and electrodes of Al thin films provided on both sidesthereof, when a triangular wave voltage having a frequency of 15 mHz andan amplitude of 120 V is applied between both electrodes, not only arectangular hysteresis curve can be obtained but also a value ofremanence polarization calculated therefrom can be not less than 75mC/m², preferably not less than 90 mC/m², more preferably not less than110 mC/m², particularly preferably not less than 120 mC/m², especiallynot less than 135 mC/m².

A substance having ferroelectricity also has properties corresponding toelectric or optical functions such as piezo electric property,pyroelectric property, electro-optical effect and non-linear opticaleffect.

Because of those properties, the thin film and laminated articleobtained in the present invention are applicable to devices using piezoelectric property, pyroelectric property, electro-optical effect andnon-linear optical effect such as FE-RAM, infrared sensor, microphone,speaker, poster with voice, head phone, electronic musical instruments,artificial tactile organ, pulsimeter, hearing aid, hemadynamometer,phonocardiograph, ultrasonic diagnostic device, ultrasonic microscope,ultrasonic hyperthermia equipment, thermograph, micro-earthquakeseismometer, landslide preperception meter, proximity warning (distancemeter) intruder detector, keyboard switch, bimorph display forunderwater communication, sonar, optical shutter, optical fibervoltmeter, hydrophone, ultrasonic light modulation and deflectiondevice, acoustic delay line, ultrasonic camera, POSFET, accelerometer,tool mulfunction sensor, AE detector, sensor for robot, impact sensor,flow meter, vibration meter, ultrasonic flaw detector, ultrasonicthickness meter, fire alarm, intruder detector, piezo-electric vidicon,copying machine, touch panel, endothermic and exothermic reactiondetector, optical intensity modulator, optical phase modulator andoptical circuit switching element.

EXAMPLE

The present invention is then explained by means of examples andpreparation examples, but is not limited to such examples.

First, methods of measuring parameters used in the present invention areexplained below.

-   [1] Method of measuring a degree of polymerization of vinylidene    fluoride (VdF) polymer-   (1) Degree of polymerization (n) of CF₃(VdF)_(n)I

Measured by ¹⁹F-NMR. Concretely calculated by the following equationusing a peak area (derived from CF₃—) around −61 ppm and a peak area(derived from —CF₂—CH₂—) around −90 to −96 ppm.(Degree of polymerization)=((Peak area around −90 to −96 ppm)/2)/((Peakarea around −61 ppm)/3)

-   (2) Degree of polymerization (n) of CF₃CF₂(VdF)_(n)I Measured by    ¹⁹F-NMR. Concretely calculated by the following equation using a    peak area (derived from CF₃-) around −86 ppm and a peak area    (derived from —CF₂—CH₂—) around −90 to −96 ppm.    (Degree of polymerization)=((Peak area around −90 to −96    ppm)/2)/((Peak area around −86 ppm)/3)-   (3) Degree of polymerization (n+m) of    I(VdF)_(n)CF₂CF₂CF₂CF₂(VdF)_(m)I

Measured by ¹⁹F-NMR. Concretely calculated by the following equationusing the sum of a peak area around −112 ppm and a peak area around −124ppm (both derived from —CF₂—CF₂CF₂CF₂) and a peak area (derived from—CF₂—CH₂—) around −90 to −96 ppm.(Degree of polymerization)=((Peak area around −90 to −96 ppm)/2)/((Sumof peak area around −112 ppm and peak area around −124 ppm)/8)

-   [2] Measuring (analysis) methods and equipment-   (1) IR analysis-   (1-1) Measuring conditions

KBr method is employed. After 1 to 5 mg of vinylidene fluoride polymerpowder is mixed to 100 to 500 mg of KBr powder and pressure is appliedfor pelletizing, the obtained pellets are fixed to a measuring equipmentand measurement is carried out at 25° C.

-   (1-2) Measuring equipment

FT-IR spectrometer 1760X available from Perkin Elmer Co., Ltd.

-   (2) ¹H-NMR and ¹⁹F-NMR analyses-   (2-1) Measuring conditions

Measurement is carried out by dissolving 10 to 20 mg of vinylidenefluoride polymer powder in d6-acetone and setting the obtained sample ona probe.

-   (2-2) Measuring equipment

AC-300P available from Brucker

-   (3) Powder X-ray diffraction analysis-   (3-1) Measuring conditions

Measurement is carried out by applying vinylidene fluoride polymerpowder on a glass plate for specific use for this analysis and settingthe glass plate on measuring equipment.

-   (3-2) Measuring equipment

Rotaflex available from Rigaku Co.

-   (4) Confirmation of ferroelectricity (D-E hysteresis curve)

When a material has ferroelectricity, a D-E hysteresis curve of thematerial shows a rectangular shape. In the present invention, electriccurrent and voltage characteristics are examined under the followingconditions and a D-E hysteresis curve is drawn to judge whether or notferroelectricity is present.

-   (4-1) Measuring conditions

A triangular wave voltage having a frequency of 15 mHz and an amplitudeof 120 V is applied on aluminum electrodes formed on both sides of VdFthin film.

-   (4-2) Measuring equipment

Electric characteristics evaluation equipment for dielectric thin filmavailable from Agilent Technologies

-   (5) Molecular weight distribution analysis-   (5-1) Measuring conditions

Measurement is carried out at 35° C. by dissolving vinylidene fluoridepolymer in THF in an amount of 0.1 to 0.2% by weight and setting onmeasuring equipment.

-   (5-2) Measuring equipment

HLC-8020 (equipment) available from Toso Kabushiki Kaisha and ShodexGPC-KF-801, GPC-KF-802 and two GPC-KF-806MX2 (columns) are used.

-   (6) Measurement of abnormal bonding ratio

Abnormal bonding ratio (%)=(n2/(n+n1+n2))×100

The abnormal bonding ratio is obtained by ¹⁹F-NMR analysis, and isconcretely calculated by the above equation from the sum of a peak areaaround −112 ppm and a peak area around −124 ppm (both derived fromabnormal bonding) (=n2) and a peak area (derived from —CF₂—CH₂—) around−90 to −96 ppm (=n+n1).

Preparation Example 1

-   (Synthesis of CF₃(VdF)_(n)I)-   (1-1) Synthesis of CF₃(VdF)_(8.1)I (n=8.1)

Into a 300 ml stainless steel autoclave equipped with a valve, pressuregauge and thermometer was poured 50 g of HCFC-225, and while coolingwith a dry ice/methanol solution, 0.78 g of di-n-propylperoxydicarbonate (50% by weight of methanol solution) was added and theinside of a system was sufficiently replaced with nitrogen gas. Afterthe inside pressure of the system was reduced, 5.2 g of CF₃I wasintroduced through the valve, and after heating of the system up to 45°C., VdF was introduced until the inside pressure of the system became0.8 MPaG. While maintaining the inside pressure and temperature of thesystem at 0.8 MPaG and 45° C., respectively, VdF was continuouslyintroduced and 9-hour reaction was carried out.

After completion of the reaction, the inside temperature of the systemwas decreased to 25° C. and the unreacted substances (VdF and CF₃I) werereleased. Then the precipitated solid reaction product (hereinafterreferred to as “VdF polymer”) was taken out and subjected to vacuumdrying in a desiccator until a constant weight was reached to obtain13.2 g of VdF polymer.

With respect to this VdF polymer, a degree (n) of polymerizationobtained by ¹⁹F-NMR analysis was 8.1. An abnormal bonding ratio was4.0%, and Mw/Mn was 1.06.

With respect to this VdF polymer, IR analysis and powder X-raydiffraction analysis were carried out. As a result, only a peak whichwas characteristic to crystal form I was recognized and it was confirmedthat the VdF polymer was one having all-I-form crystal structure (cf.FIG. 6).

-   (1-2) Synthesis of CF₃(VdF)_(5.2)I (n=5.2) Into a 300 ml stainless    steel autoclave equipped with a valve, pressure gauge and    thermometer was poured 50 g of HCFC-225, and while cooling with a    dry ice/methanol solution, 0.53 g of di-n-propylperoxy dicarbonate    (50% by weight of methanol solution) was added and the inside of a    system was sufficiently replaced with nitrogen gas. After the inside    pressure of the system was reduced, 5.4 g of CF₃I was introduced    through the valve, and after heating of the system up to 45° C., VdF    was introduced until the inside pressure of the system became 0.8    MPaG. While maintaining the inside pressure and temperature of the    system at 0.8 MPaG and 45° C., respectively, VdF was continuously    introduced and 7.5-hour reaction was carried out.

After completion of the reaction, the inside temperature of the systemwas decreased to 25° C. and the unreacted substances (VdF and CF₃I) werereleased. Then the precipitated solid reaction product (VdF polymer) wastaken out and subjected to vacuum drying in a desiccator until aconstant weight was reached to obtain 10.0 g of VdF polymer.

With respect to this VdF polymer, a degree (n) of polymerizationobtained by ¹⁹F-NMR analysis was 5.2. An abnormal bonding ratio was4.3%, and Mw/Mn was 1.08.

With respect to this VdF polymer, IR analysis and powder X-raydiffraction analysis were carried out. As a result, only a peak whichwas characteristic to crystal form I was recognized and it was confirmedthat the VdF polymer was one having all-I-form crystal structure.

-   (1-3) Synthesis of CF₃(VdF)_(10.1)I (n=10.1)

Into a 300 ml stainless steel autoclave equipped with a valve, pressuregauge and thermometer was poured 50 g of HCFC-225, and while coolingwith a dry ice/methanol solution, 0.53 g of di-n-propylperoxydicarbonate (50% by weight of methanol solution) was added and theinside of a system was sufficiently replaced with nitrogen gas. Afterthe inside pressure of the system was reduced, 5.2 g of CF₃I wasintroduced through the valve, and after heating of the system up to 45°C., VdF was introduced until the inside pressure of the system became0.8 MPaG. While maintaining the inside pressure and temperature of thesystem at 0.8 MPaG and 45° C., respectively, VdF was continuouslyintroduced and 12-hour reaction was carried out.

After completion of the reaction, the inside temperature of the systemwas decreased to 25° C. and the unreacted substances (VdF and CF₃I) werereleased. Then the precipitated solid reaction product (VdF polymer) wastaken out and subjected to vacuum drying in a desiccator until aconstant weight was reached to obtain 13.4 g of VdF polymer.

With respect to this VdF polymer, a degree (n) of polymerizationobtained by ¹⁹F-NMR analysis was 10.1. An abnormal bonding ratio was3.9%, and Mw/Mn was 1.08.

With respect to this VdF polymer, IR analysis and powder X-raydiffraction analysis were carried out. As a result, only a peak whichwas characteristic to crystal form I was recognized and it was confirmedthat the VdF polymer was one having all-I-form crystal structure.

-   (1-4) Synthesis of CF₃(VdF)_(11.0)I (n=11.0)

Into a 300 ml stainless steel autoclave equipped with a valve, pressuregauge and thermometer was poured 50 g of HCFC-225, and while coolingwith a dry ice/methanol solution, 0.38 g of di-n-propylperoxydicarbonate (50% by weight of methanol solution) was added and theinside of a system was sufficiently replaced with nitrogen gas. Afterthe inside pressure of the system was reduced, 3.5 g of CF₃I wasintroduced through the valve, and after heating of the system up to 45°C., VdF was introduced until the inside pressure of the system became0.8 MPaG. While maintaining the inside pressure and temperature of thesystem at 0.8 MPaG and 45° C., respectively, VdF was continuouslyintroduced and 9-hour reaction was carried out.

After completion of the reaction, the inside temperature of the systemwas decreased to 25° C. and the unreacted substances (VdF and CF₃I) werereleased. Then the precipitated solid reaction product (VdF polymer) wastaken out and subjected to vacuum drying in a desiccator until aconstant weight was reached to obtain 11.2 g of VdF polymer.

With respect to this VdF polymer, a degree (n) of polymerizationobtained by ¹⁹F-NMR analysis was 11.0. An abnormal bonding ratio was4.4%, and Mw/Mn was 1.13.

With respect to this VdF polymer, IR analysis was carried out. As aresult, both of peaks which were characteristic to crystal forms I andII were recognized and it was confirmed that crystal forms I and II weremixed. Further the calculated content (F(I)) of crystal form I was 85%by weight.

-   (1-5) Synthesis of CF₃(VdF)_(18.4)I (n=18.4)

Into a 300 ml stainless steel autoclave equipped with a valve, pressuregauge and thermometer was poured 50 g of HCFC-225, and while coolingwith a dry ice/methanol solution, 0.16 g of di-n-propylperoxydicarbonate (50% by weight of methanol solution) was added and theinside of a system was sufficiently replaced with nitrogen gas. Afterthe inside pressure of the system was reduced, 1.5 g of CF₃I wasintroduced through the valve, and after heating of the system up to 45°C., VdF was introduced until the inside pressure of the system became0.8 MPaG. While maintaining the inside pressure and temperature of thesystem at 0.8 MPaG and 45° C., respectively, VdF was continuouslyintroduced and 9-hour reaction was carried out.

After completion of the reaction, the inside temperature of the systemwas decreased to 25° C. and the unreacted substances (VdF and CF₃I) werereleased. Then the precipitated solid reaction product (VdF polymer) wastaken out and subjected to vacuum drying in a desiccator until aconstant weight was reached to obtain 7.9 g of VdF polymer.

With respect to this VdF polymer, a degree (n) of polymerizationobtained by ¹⁹F-NMR analysis was 18.4. An abnormal bonding ratio was3.8%, and Mw/Mn was 1.17.

With respect to this VdF polymer, IR analysis was carried out. As aresult, both of peaks which were characteristic to crystal forms I andII were recognized and it was confirmed that crystal form I and crystalform II were mixed. Further the calculated content (F(I)) of crystalform I was 18% by weight.

-   (1-6) Synthesis of CF₃(VdF)_(14.6)I (n=14.6)

Into a 300 ml. stainless steel autoclave equipped with a valve, pressuregauge and thermometer was poured 50 g of HCFC-225, and while coolingwith a dry ice/methanol solution, 0.27 g of di-n-propylperoxydicarbonate (50% by weight of methanol solution) was added and theinside of a system was sufficiently replaced with nitrogen gas. Afterthe inside pressure of the system was reduced, 2.5 g of CF₃I wasintroduced through the valve, and after heating of the system up to 45°C., VdF was introduced until the inside pressure of the system became0.8 MPaG. While maintaining the inside pressure and temperature of thesystem at 0.8 MPaG and 45° C., respectively, VdF was continuouslyintroduced and 9-hour reaction was carried out.

After completion of the reaction, the inside temperature of the systemwas decreased to 25° C. and the unreacted substances (VdF and CF₃I) werereleased. Then the precipitated solid reaction product (VdF polymer) wastaken out and subjected to vacuum drying in a desiccator until aconstant weight was reached to obtain 12.2 g of VdF polymer.

With respect to this VdF polymer, a degree (n) of polymerizationobtained by ¹⁹F-NMR analysis was 14.6. An abnormal bonding ratio was4.1%, and Mw/Mn was 1.14.

With respect to this VdF polymer, IR analysis was carried out. As aresult, both of peaks which were characteristic to crystal forms I andII were recognized and it was confirmed that crystal form I and crystalform II were mixed. Further the calculated content (F(I)) of crystalform I was 60% by weight.

-   (1-7) Synthesis and separation of CF₃(VdF)₃I (n=3)

Into a 300 ml stainless steel autoclave equipped with a valve, pressuregauge and thermometer was poured 500 g of HCFC-225, and while coolingwith a dry ice/methanol solution, 21 g of di-n-propylperoxy dicarbonate(50% by weight of methanol solution) was added and the inside of asystem was sufficiently replaced with nitrogen gas. After the insidepressure of the system was reduced, 200 g of CF₃I was introduced throughthe valve, and after heating of the system up to 45° C., VdF wasintroduced until the inside pressure of the system became 0.8 MPaG.While maintaining the inside pressure and temperature of the system at0.8 MPaG and 45° C., respectively, VdF was continuously introduced and3.5-hour reaction was carried out.

After completion of the reaction, the inside temperature of the systemwas decreased to 25° C. and the unreacted substances (VdF and CF₃I) werereleased. Then the precipitated solid reaction product was filtrated offand a filtrate was subjected to fractional distillation under reducedpressure (5 mmHg). The distillate of 55° C. was analyzed by ¹⁹F-NMRanalysis and the degree (n) of polymerization of the distillate of 55°C. was 3. The polymer of n=3 was in the form of liquid at 25° C.

-   (1-8) Synthesis of mixture of crystal form I of CF₃(VdF)_(8.1)I    (n=8.1) and crystal form III

3 g of the VdF polymer powder having all-I-form crystal structure ofCF₃(VdF)_(8.1)I (n=8.1) synthesized in (1-1) above was put in a petridish, and the dish was placed in a desiccator. The powder was heated at200° C. for one hour and completely melted. Then the dish was taken outfrom the desiccator and allowed to stand at 25° C. for rapid cooling to25° C.

With respect to the obtained VdF polymer, IR analysis was carried out.As a result, both of peaks which were characteristic to crystal form Iand crystal form III were recognized and it was confirmed that crystalform I and crystal form III were mixed. Further the calculated content(F(I)) of crystal form I was 67% by weight (cf. FIG. 7).

Preperation Example 2

-   (Synthesis of CF₃CF₂(VdF)_(n)I)-   (2-1) Synthesis of CF₃CF₂(VdF)_(10.9)I (n=10.9)

Into a 300 ml stainless steel autoclave equipped with a valve, pressuregauge and thermometer was poured 50 g of HCFC-225, and while coolingwith a dry ice/methanol solution, 0.08 g of di-n-propylperoxydicarbonate (50% by weight of methanol solution) was added and theinside of a system was sufficiently replaced with nitrogen gas. Afterthe inside pressure of the system was reduced, 1.96 g of CF₃CF₂I wasintroduced through the valve, and after heating of the system up to 45°C., VdF was introduced until the inside pressure of the system became0.8 MPaG. While maintaining the inside pressure and temperature of thesystem at 0.8 MPaG and 45° C., respectively, VdF was continuouslyintroduced and 9-hour reaction was carried out.

After completion of the reaction, the inside temperature of the systemwas decreased to 25° C. and the unreacted substances (VdF and CF₃CF₂I)were released. Then the precipitated solid reaction product (VdFpolymer) was taken out and subjected to vacuum drying in a desiccatoruntil a constant weight was reached to obtain 7.3 g of VdF polymer.

With respect to this VdF polymer, a degree (n) of polymerizationobtained by ¹⁹F-NMR analysis was 10.9. Also Mw/Mn was 1.10.

With respect to this VdF polymer, IR analysis was carried out. As aresult, both of peaks which were characteristic to crystal forms II andIII were recognized and it was confirmed that crystal form II andcrystal form III were mixed. Further the calculated content (F(III)) ofcrystal form III was 57% by weight (cf. FIG. 8).

Preparation Example 3

-   (Synthesis of I(VdF)_(n)C₄F₈(VdF)_(m)I)-   (3-1) Synthesis of I(VdF)_(n)(CF₂CF₂)₂(VdF)_(m)I (n+m=8.7)

Into a 300 ml stainless steel autoclave equipped with a valve, pressuregauge and thermometer was poured 50 g of HCFC-225, and while coolingwith a dry ice/methanol solution, 0.27 g of di-n-propylperoxydicarbonate (50% by weight of methanol solution) was added and theinside of a system was sufficiently replaced with nitrogen gas. Afterthe inside pressure of the system was reduced, 1.96 g of I(CF₂CF₂)₂I wasintroduced through the valve, and after heating of the system up to 45°C., VdF was introduced until the inside pressure of the system became0.8 MPaG. While maintaining the inside pressure and temperature of thesystem at 0.8 MPaG and 45° C., respectively, VdF was continuouslyintroduced and 9-hour reaction was carried out.

After completion of the reaction, the inside temperature of the systemwas decreased to 25° C. and the unreacted substances (VdF andI(CF₂CF₂)₂I) were released. Then after the precipitated solid reactionproduct (VdF polymer) was taken out by filtration and washed withHCFC-225, the product was subjected to vacuum drying in a desiccatoruntil a constant weight was reached to obtain 8.8 g of VdF polymer.

With respect to this VdF polymer, a degree (n+m) of polymerizationobtained by ¹⁹F-NMR analysis was 8.7. Also Mw/Mn was 1.03.

With respect to this VdF polymer, IR analysis was carried out. As aresult, both of peaks which were characteristic to crystal forms I andII were recognized and it was confirmed that crystal form I and crystalform II were mixed. Further the calculated content (F(I)) of crystalform I was 79% by weight (cf. FIG. 9).

-   (3-2) Synthesis of I(VdF)_(n)(CF₂CF₂)₂(VdF)_(m)I (n+m=10.4)

Into a 300 ml stainless steel autoclave equipped with a valve, pressuregauge and thermometer was poured 50 g of HCFC-225, and while coolingwith a dry ice/methanol solution, 0.162 g of di-n-propylperoxydicarbonate (50% by weight of methanol solution) was added and theinside of a system was sufficiently replaced with nitrogen gas. Afterthe inside pressure of the system was reduced, 3.5 g of I(CF₂CF₂)₂I wasintroduced through the valve, and after heating of the system up to 45°C., VdF was introduced until the inside pressure of the system became0.8 MPaG. While maintaining the inside pressure and temperature of thesystem at 0.8 MPaG and 45° C., respectively, VdF was continuouslyintroduced and 9-hour reaction was carried out.

After completion of the reaction, the inside temperature of the systemwas decreased to 25° C. and the unreacted substances (VdF andI(CF₂CF₂)₂I) were released. Then after the precipitated solid reactionproduct (VdF polymer) was taken out by filtration and washed withHCFC-225, the product was subjected to vacuum drying in a desiccatoruntil a constant weight was reached to obtain 7.2 g of VdF polymer.

With respect to this VdF polymer, a degree (n+m) of polymerizationobtained by ¹⁹F-NMR analysis was 10.4. Also Mw/Mn was 1.04.

With respect to this VdF polymer, IR analysis was carried out. As aresult, both of peaks which were characteristic to crystal forms I andII were recognized and it was confirmed that crystal form I and crystalform II were mixed. Further the calculated content (F(I)) of crystalform I was 70% by weight.

Preparation Example 4

-   (4-1) Synthesis of CF₃(VdF)_(n)C₂H₄I (n=8.1)

Into a 300 ml stainless steel autoclave equipped with a valve, pressuregauge and thermometer were poured 3 g of vinylidene fluoride oligomer(n=8.1) synthesized in (1-1) of Preparation Example, 30 g of ethylacetate and 0.034 g of AIBN while the temperature inside a system wasmaintained at 25° C., and the inside of the system was sufficientlyreplaced with nitrogen gas. After reducing the inside pressure of thesystem and heating the system up to 65° C., ethylene gas was introduceduntil the inside pressure of the system became 0.7 MPaG. Whilemaintaining the inside pressure and temperature of the system at 0.7MPaG and 65° C., respectively, ethylene gas was continuously introducedand 5-hour reaction was carried out.

After completion of the reaction, the inside temperature of the systemwas decreased to 25° C. and the unreacted ethylene gas was released.Then the ethyl acetate solution in the system was poured into hexane andthe precipitated solid reaction product (hereinafter referred to as“vinylidene fluoride oligomer/ethylene adduct”) was taken out byfiltration. The vinylidene fluoride oligomer/ethylene adduct wassubjected to vacuum drying in a desiccator until a constant weight wasreached, and 2.7 g of the adduct was obtained.

According to ¹H-NMR and ¹⁹F-NMR analyses of this vinylidene fluorideoligomer/ethylene adduct, it was recognized that the peak around −38 ppmderived from the end —CF₂I had been disappeared and peaks derived fromthe added ethylene were observed around 3.4 to 3.2 ppm and 2.8 to 2.6ppm by 1H-NMR. In this case, an end modification ratio obtained by¹H-NMR was 95%.

According to IR analysis and powder X-ray diffraction analysis, only apeak which was characteristic to crystal form I was recognized and itwas confirmed that the adduct was one having all-I-form crystalstructure.

-   (4-2) Synthesis of CF₃(VdF)_(n)C₂H₅ (n=8.1)

Into a 50 ml four-necked flask equipped with a reflux condenser,thermometer, stirrer and dropping funnel were poured 30 ml of aceticacid, 0.5 g of vinylidene fluoride oligomer/ethylene adduct:CF₃(VdF)_(n)C₂H₄I (n=8.1) synthesized in (4-1) of Preparation Exampleand 0.53 g of zinc powder, and 4-hour refluxing with heating was carriedout.

After completion of the reaction, the inside temperature of a system wasdecreased to 25° C. and the zinc powder was removed by filtration. Thenthe reaction product, i.e. acetic acid solution was poured into purewater and the solid reaction product was taken out by re-precipitation.The solid reaction product was subjected to vacuum drying in adesiccator until a constant weight was reached, and 0.32 g of theproduct was obtained.

According to ¹H-NMR analysis of this solid reaction product, it wasrecognized that the peaks derived from ethylene around 3.4 to 3.2 ppmand 2.8 to 2.6 ppm had been disappeared, the peak derived from the endmethyl group was observed around 1.1 to 0.8 ppm, and the end iodine ofthe vinylidene fluoride oligomer/ethylene adduct had been converted toproton. In this case, an end modification ratio obtained by ¹H-NMR was96%.

According to IR analysis and powder X-ray diffraction analysis, only apeak which was characteristic to crystal form I was recognized and itwas confirmed that the adduct was one having all-I-form crystalstructure.

Example 1

(Formation of Ferroelectric Thin Film of VdF Polymer Having All-I-formCrystal Structure by Spin Coating Method)

The CF₃(VdF)_(10.1)I polymer having all-I-form crystal structureprepared in (1-3) of Preparation Example 1 was dissolved in acetone tomake 3% by weight of acetone solution. The acetone solution was appliedon an aluminum electrode at a rotation speed of 2,000 rpm by spincoating method to form a thin film, and then the solvent was distilledoff in a desiccator. Thus a 200 nm thick thin film of VdF polymer havingall-I-form crystal structure was formed.

The spin coating was carried out under the following condition by usingthe following equipment.

-   Coating condition

Number of revolutions: 2,000 rpm

-   Equipment

MIKASA SPINCOATER 1H-D7 available from Mikasa

-   Kabushiki Kaisha

Then vacuum vapor deposition of aluminum was carried out by usual methodon the thin film of VdF polymer having all-I-form crystal structurewhich was formed on the aluminum electrode, thereby forming a secondelectrode.

The obtained laminated article was subjected to polarization treatmentunder the following conditions.

-   Thin film temperature: 25° C.-   Applied voltage: 200 MV/m-   Treating time: 30 minutes

With respect to the thin film of VdF polymer having all-I-form crystalstructure subjected to polarization treatment, electricalcharacteristics were evaluated, and as a result, the obtained D-Ehysteresis curve showed a rectangular shape specific to ferroelectricmaterials.

Example 2

(Formation of Ferroelectric Thin Film of VdF Polymer Having All-I-formCrystal Structure by Vacuum Vapor Deposition Method)

A 200 nm thick thin film of VdF polymer having all-I-form crystalstructure was formed on an aluminum electrode by vacuum vapor depositionmethod by using powder of CF₃(VdF)_(10.1)I polymer having all-I-formcrystal structure prepared in (1-3) of Preparation Example 1.

The vacuum vapor deposition was carried out under the followingcondition by using the following equipment.

-   Deposition condition

Substrate temperature: 25° C.

-   Equipment

Organic thin film forming equipment available from Jyonan

-   Kogyo Kabushiki Kaisha

Then vapor deposition of aluminum was carried out by usual method on thethin film of VdF polymer having all-I-form crystal structure which wasformed on the aluminum electrode, thereby forming a second electrode.The obtained laminated article was subjected to polarization treatmentunder the same conditions as in Example 1.

With respect to the thin film of VdF polymer having all-I-form crystalstructure subjected to polarization treatment, electricalcharacteristics were evaluated, and as a result, the obtained D-Ehysteresis curve showed a rectangular shape specific to ferroelectricmaterials.

According to the present invention, there is provided a method offorming a ferroelectric thin film of vinylidene fluoride homopolymerhaving crystal form I which is applicable to various substrates inrelatively easy way (coating conditions, application method, etc.).

1. A method of forming a ferroelectric thin film comprising vinylidenefluoride homopolymer which comprises the following steps (i), (ii) and(iii). (i) A step for preparing a green powder product of vinylidenefluoride homopolymer comprising crystal form I alone or as maincomponent by subjecting vinylidene fluoride to radical polymerization inthe presence of a radical polymerization initiator. (ii) A step forforming a thin film on a substrate surface by using vinylidene fluoridehomopolymer which comprises crystal form I alone or as main componentand is obtained from the green powder product of vinylidene fluoridehomopolymer comprising crystal form I alone or as main component. (iii)A step for subjecting the thin film of vinylidene fluoride homopolymerformed in the step of above (ii) to polarization treatment.
 2. Themethod of forming a ferroelectric thin film of claim 1 which includes astep (iv) for heat-treating the thin film of vinylidene fluoridehomopolymer at a temperature of not less than 50° C. and lower than acrystalline melting point of the vinylidene fluoride homopolymer.
 3. Themethod of forming a ferroelectric thin film of claim 1, wherein in thevinylidene fluoride homopolymers comprising crystal form I alone or asmain component, when attention is given to proportions of the respectivevinylidene fluoride homopolymers having crystal forms I, II or III inthe green powder product of vinylidene fluoride homopolymer which arecalculated by IR analysis, the proportion of vinylidene fluoridehomopolymers having crystal form I satisfies both of (Equation 1):100≧I-form/(I-form+II-form)≧50% by weight  (Equation 1) and (Equation2):100≧I-form/(I-form+III-form)≧50% by weight  (Equation 2).
 4. The methodof forming a ferroelectric thin film of claim 3, wherein the proportionof vinylidene fluoride homopolymers having crystal form I satisfies bothof (Equation 3):100≧I-form/(I-form+II-form)≧70% by weight  (Equation 3) and (Equation4):100≧I-form/(I-form+III-form)≧70% by weight  (Equation 4).
 5. The methodof forming a ferroelectric thin film of claim 1, wherein a numberaverage degree of polymerization of the green powder product ofvinylidene fluoride homopolymer comprising crystal form I alone or asmain component is from 4 to
 20. 6. The method of forming a ferroelectricthin film of claim 1, wherein the step (ii) for forming a thin film ofvinylidene fluoride homopolymer is carried out by applying a liquidcomposition containing the vinylidene fluoride homopolymer on asubstrate surface.
 7. The method of forming a ferroelectric thin film ofclaim 1, wherein the step (ii) for forming a thin film of vinylidenefluoride homopolymer is carried out by vacuum vapor deposition of acomposition containing the vinylidene fluoride homopolymer on asubstrate surface.
 8. The method of forming a ferroelectric thin film ofclaim 6, wherein the step (ii) for forming a thin film of vinylidenefluoride homopolymer is carried out at a temperature of not less than10° C. and lower than a crystalline melting point of the vinylidenefluoride homopolymer.
 9. The method of forming a ferroelectric thin filmof claim 7, wherein the step (ii) for forming a thin film of vinylidenefluoride homopolymer is carried out at a temperature of not less than10° C. and lower than a crystalline melting point of the vinylidenefluoride homopolymer.
 10. The method of forming a ferroelectric thinfilm of claim 1, wherein the thin film of vinylidene fluoridehomopolymer is formed on a surface of silicon substrate.
 11. The methodof forming a ferroelectric thin film of claim 1, wherein the thin filmof vinylidene fluoride homopolymer is formed on a surface of metallicsubstrate.
 12. The method of forming a ferroelectric thin film of claim11, wherein the metallic substrate is at least one selected from thegroup consisting of aluminum, copper, gold, silver and platinum.
 13. Themethod of forming a ferroelectric thin film of claim 1, wherein thevinylidene fluoride homopolymer comprising crystal form I alone or asmain component is applied on a substrate surface in the form of thinfilm in a thickness of 0.1 to 1,000 nm.
 14. The method of forming aferroelectric thin film of claim 1, wherein the polarization is carriedout by poling treatment.